Device for forming an X-ray or gamma beam of small cross-section and variable direction

ABSTRACT

A device for forming an X-ray beam or gamma beam (11) having a small cross-section and a variable direction, includes an X-ray source or gamma source (1) which supplies an X-ray beam and a diaphragm device which forms the X-ray beam from the radiation beam. The diaphragm device has a stationary diaphragm section (7) provided with a rectilinear slit (8) and a cylindrical first diaphragm body (3) which rotates about an axis of rotation (5) and which is provided with a helical slit (9) on its outer surface. In order to reduce the expenditure for manufacturing a device which is also suitable for different distances between the radiation source and the axis of rotation, the diaphragm body (3) has an at least approximately semi-circular cross-section over at least a part of its length.

BACKGROUND OF THE INVENTION

The invention relates to a device for forming an X-ray or gamma ray beamof small cross-section and variable direction, comprising an X-raysource or gamma source which supplies a radiation beam and a diaphragmdevice which forms the X-ray beam from the radiation beam and whichcomprises a stationary diaphragm section provided with a rectilinearslit and a cylindrical first diaphragm body which rotates about an axisof rotation and which is provided with a helical slit on its outersurface.

Such devices are known from EP-OS No. 74 021 for medical applicationsand from, which corresponds to U.S. Pat. No. 4,750,196, for industrialapplications. Therein, the diaphragm body is formed by a hollow cylinderof a radiation absorbing material, the circumference of which isprovided with two mutually offset helically extending slits. When a beamof parallel rays is incident on such a diaphragm body in the directionperpendicular to the cylinder axis thereof, there will always be onepoint in which the X-ray beam passes through both slits. When thediaphragm body is rotated, this point travels along the axis so that aperiodically moving X-ray beam emerges behind the diaphragm body. Thisperiodically displaced X-ray beam can be used for medical or industrialexaminations. The stationary diaphragm section serves to bound theX-rays perpendicularly to the direction of displacement of the X-raybeam in a defined manner.

In practice X-rays are produced by means of an X-ray tube which deliversa beam of diverging rays. In a diverging radiation beam, however, theintensity of the X-ray beam decreases towards the edges. This can be atleast partly prevented by suitably shaping the slits and by ensuringthat instead of extending exactly parallel to the plane formed by therectilinear slit and the radiation source, the axis of rotation of thediaphragm body encloses an angle with respect thereto which angledepends on the divergence of the radiation beam, i.e. on the distancebetween the radiation source and the axis of rotation of the diaphragmbody.

Therefore, such a diaphragm body can be manufactured only atconsiderable expenditure and is suitable only for one given distancebetween the radiation source and the axis of rotation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a device of the kindset forth which produces a reciprocating primary beam in a differentmanner.

This object is achieved in accordance with the invention in that thediaphragm body has an at least approximately semi-circular cross-sectionover at least a part of its length.

Thus, in accordance with the invention the X-ray beam is formed from the(diverging) X-ray beam by cooperation of a helical slit in the firstdiaphragm body and a rectilinear slit in a stationary diaphragm section.The point of emergence of the X-ray beam is displaced by rotation of thefirst diaphragm body, resulting in a periodically moving X-ray beam.Because the X-rays emerge through a slit instead of through twooppositely situated slits in the diaphragm body, the intensity of theX-ray beam does not decrease towards the ends of the diaphragm body.Therefore, it is not necessary to impart a given shape to the slit or toarrange it in a defined manner with respect to the rectilinear slit.

The cooperation of the slits in the stationary diaphragm section and thediaphragm body defines an X-ray beam having a trapezoidal cross-section.However, it is desirable to obtain a square (or circular) cross-sectionwhich would produce a direction-independent spatial resolution. For thesame width of the two slits, the approximation of the squarecross-section is better as the angle at which the projection of the twoslits intersect one another becomes larger, e.g. approaches 90°. Alarger angle of intersection could be achieved by using a diaphragm bodyhaving a large diameter and a small axial length. For many applications,however, a comparatively large deflection angle is required for theX-ray beam, implying a corresponding axial length of the diaphragm body;because of the inherently large volume, a large diameter is undesirablefor many applications.

In order to achieve an attractive beam cross-section also in the case ofa diaphragm body having a comparatively large axial length and acomparatively small diameter, in a further version of the invention aplurality of helical circumferential slits are provided in the firstdiaphragm body so that they overlap one another in the axial direction.In the case of n slits in the first diaphragm body, each slit extendsover only 1/n of the length of the diaphragm body, so that theprojection of the helical slits in the diaphragm body on the diaphragmsection intersect the slit provided therein each time at a comparativelylarge angle, resulting in an attractive cross-sectional shape.

The n helical slits produce n X-ray beams which, in response to one halfrevolution of the diaphragm body, travel each time to a position whichwas occupied by a neighbouring X-ray beam at the beginning. Fordifferent applications, for example for applications where the scatteredradiation produced by the X-ray beam is measured as described in DE OSNo.34 43 095, however, the use of a single X-ray beam only is desired.

A further version of the invention which is suitable for this purpose ischaracterized in that there is provided a second diaphragm body whichhas a semicircular cross-section over at least a part of its lenght, thetwo diaphragm bodies being coaxially arranged so that one encloses theother, the first diaphragm body rotating faster as a function of thenumber of slits provided therein with respect to the second diaphragmbody, the arrangement and the shape of the aperture (apertures) on thecircumference of the second diaphragm body being such that a usable beamcan emerge each time from only one of the slits (for example 9b).

In the simplest version of this embodiment in accordance with theinvention a helical slit is provided as the single aperture in thesecond diaphragm body, which slit is substantially wider than the slitsin the first diaphragm body and extends over a circumferential angle ofat least approximately 180°. The slits are thus successively traversedin the course of one half revolution of the second diaphragm body, theX-ray beam continuously moving from one side to the other. Because thenumber of revolutions of the first diaphragm body is higher than that ofthe second diaphragm body, one slit in the first diaphragm body will betraversed when the slit or the diaphragm body faces the radiationsource, a neighbouring slit being traversed when it or the diaphragmbody is remote from the radiation source. The X-ray beam then producedis wider in the first case than in the second case, the differencesbeing greater as the diameter of the diaphragm body is greater withrespect to its distance from the radiation source.

In order to prevent the X-ray beam from being alternately wider andnarrower in cases where the above condition is not satisfied in afurther embodiment in accordance with the invention n apertures areprovided in the second diaphragm body, n being the number of slits inthe first diaphragm body, the apertures being offset through an angle of180°/n on the circumference, their axial position corresponding to theaxial position of each time one slit so that the radiation passes eachtime through the slit via the associated aperture.

In this respect it is assumed that the X-rays are switched off each timewhen the slits in the first diaphragm body face the radiation source.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in detail hereinafter with reference todrawings. Therein:

FIG. 1 shows a first embodiment in accordance with the invention,

FIG. 2 shows the diaphragm body and the diaphragm section of a firstembodiment,

FIG. 3 shows the diaphragm body and the diaphragm section in a secondembodiment,

FIG. 4 is a cross-sectional view of a preferred embodiment,

FIGS. 5 and 6 are plan views of the first and the second diaphragm body,respectively, of the embodiment shown in FIG. 4, and

FIGS. 7 and 8 show a development of the first and the second diaphragmbody, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The reference numeral 1 in FIG. 1 denotes the focus of an X-ray tube,(not shown) which emits an X-ray beam 2 which is denoted by brokenlines. The X-ray beam 2 is incident on a diaphragm body 3 which isformed by a hollow cylinder having a semi-circular cross-section. At itsend faces the diaphragm body is supported by circular discs 4. The discs4 are rotatable about an axis of rotation 5 by a motor (not shown) onwhich the centres of curvature of the inner surface of the hollowcylinder 3 are situated. As is denoted by the broken line 6, the discs 4are immobile in the axial direction. This can be achieved, for exampleby journalling the discs 4 so as to be rotatable about the axis 5 in arespective bearing which is arranged in a housing (not shown) which isconnected to the X-ray source.

The diaphragm body 3 is made of such a material and has such a thicknessthat the X-rays are substantially completely absorbed thereby. Thethickness and the material of the diaphragm body depend on the intensityof the X-rays. For an X-ray tube voltage of 120 kV, a diaphragm bodyconsisting of tungsten or a tungsten alloy and having a thickness of 1.5mm absorbs the X-rays substantially completely. The thickness of thebody 3 is limited to a value d_(m) which results from the relation

    d.sub.m =w/tan (β/2),

where β is the deflection angle of the X-ray beam and w is the width ofthe X-ray beam. For a deflection angle of, for example 23° and w=0.5 mm,d_(m) =2.45 mm.

The diaphragm body 3 is provided with a helical slit 9 which connectsone corner of the diaphragm body (top right) to the oppositely situatedcorner (bottom left). The width of the slit is adapted to the size ofthe X-ray beam to be formed.

On the other side of the diaphragm body (viewed from the focal point 1)there is arranged a stationary diaphragm plate 7 which comprises arectilinear slit 8. The radiation source 1, the diaphragm body 3 and thediaphragm plate 7 are preferably so arranged with respect to one anotherthat the axis of rotation 5 of the diaphragm body is situated in theplane defined by the central line of the slit 8 and the focus 1 andextends parallel to the plane defined by the diaphragm 7. The focus 1should be situated as well as possible in the symmetry plane of thediaphragm body 3 and the diaphragm plate 7, which plane extendsperpendicularly with respect to the axis of rotation.

The X-ray beam 2 passes through the slit 9 in the diaphragm body at thearea where the slit 9 is intersected by the plane defined by the axis ofrotation 5 and the focus 1, as well as at a given area around this pointof intersection. The diaphragm plate 7, however, transmits only theX-ray beam emerging from the point of intersection and substantiallysuppresses the X-rays emerging on both sides of this beam from the slit9, i.e. above and below the plane defined by focus 1 and slit 8, so thatan X-ray beam 11 having a small cross-section (pencil beam) emerges fromthe slit 8. Between source 1 and the device comprising body 3 and plate7, there is arranged a diaphragm (not shown) which prevents radiationfrom emerging while by-passing the diaphragm device.

In response to a rotation of the diaphragm body 3 about the axis 5 inthe direction denoted by the arrow 10, the point of intersection betweenthe plane defined by focus 1 and slit 8 and the slit 9 is displaced tothe right, so that the X-ray beam 11 also travels to the right until theend of the slit 9 is reached. When the diaphragm body 3 is rotatedfurther, the X-ray beam first traverses the left hand end of the slit 9and subsequently moves to the right again, so that a periodicsawtooth-shaped motion of the X-ray beam is obtained when the diaphragmbody is rotated at a constant speed.

The period of time elapsing between the end of the motion (right) andthe beginning anew (left) on the one hand depends on the circumferentialangle described by the slit 9 on the diaphragm body and on the otherhand on the arc described by the cross-section of the body itself. Ifthe period is to be short, the arc may be only slightly larger than 180°and the angle described by the slit 9 on the circumference of thediaphragm body must be 180°. The arc may not be made exactly equal to180° because in the angular position of the diaphragm body in which thediaphragm body 3 edge occupies the angular position defined by the axisof rotation 5 and the focus 1, X-rays would emerge from the slit 8 whileby-passing the diaphragm body 3. Consequently, the arc of the circularcross-section of the diaphragm body 3 must be slightly larger than 180°,so that the slit 8 is always shielded from the focus 1 by the diaphragmbody 3 until the position is reached in which the X-ray beam 11traverses the slit 9.

It has been assumed thus far that the slit 9 has a helical shape, i.e.that its pitch is constant over the entire length of the body 3. In manycases, however, it is desirable that the beam 11 is moved faster (orslower) at the centre than at the edge. Such an embodiment is shown inFIG. 2 which shows the stationary diaphragm plate 7 and the diaphragmbody 3 from side of the diaphragm plate 7. It appears that the pitch ofthe slit (viewed in the axial direction) is greater in the centre thanat the edge. Consequently, in the case of a constant rotary speed of thediaphragm body, the X-ray beam moves faster at the centre than at theedges of the slit 9.

FIG. 3 shows an embodiment in which the slit 9 is formed by two steppedcurves 9' and 9, the width-to-height ratio being the same for all stepsand corresponding to the ratio of the length of the diaphragm body toits circumference. In this embodiment the X-ray beam formed is not movedcontinuously but intermittently. This is advantageous for variousapplications, for example for applications where the intensity of theprimary beam (on the other side of the examination zone) is measured bymeans of a plurality of adjacently arranged detectors.

When the diaphragm body forms an integral unit, the angle described bythe cross-sectional arc of the diaphragm body must always be larger (formechanical reasons) than the circumferential angle described by theslit. This prolongs the period of time elapsing between thedisappearance of the X-ray beam (at the right hand end) and itsre-emergence (at the left hand edge). Moreover, it is comparativelycomplex to form a slit in an integral diaphragm body, notably when astepped shape as shown in FIG. 3 is required.

These drawbacks can be avoided by manufacturing the diaphragm body fromtwo matching sections which are separated by the slit and whose endfaces are rigidly connected to the discs 4 in which they are preferablyinserted into segment shaped grooves provided therein. The desiredcourse of the slit 9 can then be comparatively simply realised bysuitable working of the facing surfaces of the two sections forming thediaphragm body.

Because the central projection (as from the focus 1) of the slit 9 onthe diaphragm plate 7 intersects the slit 8 at an angle other than 90°,the cross-section of the X-ray beam 11 will be greater in the directionof the slit than in the direction perpendicular thereto, even though thewidth of the slits 8 and 9 is the same. This difference is greater asthe diameter of the diaphragm body 3 is smaller and its length isgreater.

A more distinct lateral delimitation could be achieved by tilting theaxis 5 out of the plane defined by the focus 1 and the slit 8, i.e. sothat the right hand side of the diaphragm body is raised and the lefthand side is lowered. This is because the angle between the centralprojection of the slit 9 and the slit 8 would then be larger so that theX-ray beam 11 would be somewhat more strictly defined in the lateraldirection. However, the arc of the diaphragm body 3 should then beprolonged even further beyond 180° in order to prevent X-rays fromreaching the slit while bypassing the diaphragm body in a givenposition.

FIG. 4 shows a preferred embodiment which produces a substantiallybetter lateral delimitation of the X-ray beam 11 formed for the samelength and the same diameter of the diaphragm body. Instead of only onediaphragm body, between the radiation source 1 and the diaphragm section7 there are arranged a first diaphragm body 3 and a second diaphragmbody 12, which bodies are coaxially arranged so that one enclose theother. The two diaphragm bodies again have a semi-circular cross-sectionover at least a part of their length which may amount to, for example 50mm. Like in the device shown in FIG. 1, between the radiation source 1and the diaphragm device there is arranged a diaphragm (not shown) whichprevents radiation from emerging while by-passing the diaphragm device.

The first diaphragm body 3, enclosing the second diaphragm body 12, isshown in FIG. 5 as well as in a development in FIG. 7. This firstdiaphragm body comprises 5 slits 9a . . . 9e which have a width of, forexample 0.4 mm. However, a different number of slits may also beprovided, be it that this number be odd. As appears from FIG. 7, theslits 9a . . . 9e all have the same, constant pitch. In the axialdirection each of the slits extends over one fifth of the length of thezone of semi-circular cross-section. A given overlap then exists betweenthe areas occupied by the slits in the axial direction, i.e. one slitalready commences when the neighbouring slit has not yet completelyterminated. Thus, all positions within the deflection angle of the X-raybeam, determined by the construction, are irradiated by a beam 8. Thecross-section through the first body 3 describes an arc of circle ofexactly 180°. In order to ensure that the slits do not slice up thediaphragm body, they extend through a circumferential angle which isslightly smaller than 180°, for example 170°, as appears from FIG. 7.

FIGS. 6 and 8 show that the inner, second diaphragm body 12 which isshown in a plan view and a development, respectively, comprisestrapezoidal apertures 13a . . . 13e whose number corresponds to thenumber of slits in the first diaphragm body; in the example, therefore,there are five apertures. Each aperture extends in the axial directionacross a region whose central projection on the first diaphragm body 3has a length which at least equals that of a slit in the first diaphragmbody (in the axial direction). Each of the apertures 13a . . . 13eextends across a circumferential angle which is wider than one fifth ofthe circumferential angle described by the slits 9a . . . 9e in thefirst diaphragm body, for example through 36°. The apertures are offseton the circumference by 180°/5, i.e. by 36°. The aperture 13a associatedwith the slit 9a extends through an angular range of from 0° to 36°. Theaperture 13b associated with the second slit 9b extends through theangular range from 72° to 108° and the third aperture 13c which isassociated with the slit 9c extends through an angular range of from144° to 180°. The aperture 13d covers the angular range from 36° to 72°and the aperture 13e covers the range from 108° to 144°. Portions 14 and15 at the area of the outer apertures 13a and 13c close off theapertures from the surroundings.

The outer diameter of the diaphragm bodies 3 and 12 may amount to, forexample, 12.5 mm and 8.4 mm, respectively, and their axial length inwhich slits or apertures are provided amounts to approximately 50 mm. Itis to be noted that the FIGS. 4, 5, 6, 7, 8 show these diaphragm bodieseach time at a different scale.

During operation, the diaphragm bodies rotate in the same direction, theangular speed of the first diaphragm body being five times higher thanthat of the second diaphragm body. This can be achieved by providing theend faces of the diaphragm bodies with toothed portions (not shown)which are coupled to a common drive motor, the gear ratio being chosenaccordingly for the two diaphragm bodies. Instead, however, for eachdiaphragm body there may also be provided a separate stepping motor, themotor for the first diaphragm body receiving five times as many drivepulses for the same step width, or the same number of pulses when thestep width is five times larger.

It is assumed that at a given instant during operation the slits 9a . .. 9e face the diaphragm plate 7 and that the focal spot is situated in aplane which extends perpendicularly to the plane of drawing and whichcontains the axis of rotation 5. In this case an X-ray beam can passthrough the aperture 13b, the slit 9b and the slit 8 in the diaphragmplate 7. All other slits (more accurately speaking, the parts of theseslits which are situated in the plane defined by the source 1 and theaxis 5) are shielded by the diaphragm body 12. Under the influence ofthe rotation, the point of intersection of the slit 9b and the latterplane is shifted to the right, i.e. the X-ray beam travels to the right,the aperture 13b being moved upwards at the same time until the end ofthe slit 9b is reached. The X-ray beam then emerges just from the lowerarea of the aperture 13b.

When the end of the slit 9b has been reached in this manner, the beamthrough the slit 9b is interrupted by further rotation and the frontedge (with respect to the direction of rotation) of the first diaphragmbody 3 intersects the latter plane. Subsequently, radiation passesthrough another slit, that is to say the slit whose associated aperturesucceeds the aperture in the circumferential direction which has lastbeen irradiated. In the present embodiment, this would be the slit 9eand the aperture 13e. However, the slit 9e would then face the radiationsource 1, i.e. it would be situated between the radiation source and theaxis 5, while the slit 9b was previously situated on the other side ofthe axis 5. For an equal slit width, the cross-section of the X-ray beamwould thus be increased. This variation of the width of the X-ray beamformed can itself still have a disturbing effect when the distancebetween the radiation source 1 and the axis 5 amounts to, for example110 mm in the case of an outer diameter of the diaphragm body 3 of 12.5mm.

In order to avoid the described widening of the X-ray beam, the X-raysare switched off until the front edge of the diaphragm body 3 issituated underneath the plane defined by focus 1 and axis 5. Switchingoff can be controlled by an angle detector (not shown) which is coupledto the diaphragm body 3.

When the front edge of the diaphragm body 3 passes through the latterplane in the upwards direction, the upper edge of the aperture 13c isalso situated in this plane. This aperture subsequently passes the beampassed by the slit 9c and the X-ray beam emerging through this slitagain moves to the right. Because of the described overlapping of theslits 9a . . . 9e in the axial direction, the X-ray beam emerges at thesame position, with respect to the plane, in which it emerged uponreaching the end of the slit 9b, or slightly to the left thereof. As aresult, the dose of the X-ray beam in the overlapping zone could behigher than that outside this zone because the X-rays at the end of theslit 9b and at the beginning 9c are summed. This effect, however, can becompensated for when in this position of the X-ray beam defined by theangle detector, the measurement values of the detector device (notshown) which detects the X-rays transmitted or scattered by an object(not shown) are multiplied by a suitable weighting factor. Subsequently,the X-ray beam moves from left to right in the range defined by the slit9c.

Subsequently, the X-rays are switched off again, after which theaperture 13d and the slit 9d become operative. However, if the aperture13c were previously situated on the other side (with respect to theradiation source 1) of the axis 5, the aperture 13d will now be situatedbetween the axis 5 and the radiation source 1. The projection of theaperture 13d on the diaphragm body 3 is thus increased, but does nothave a disturbing effect because the cross-section of the X-ray beamformed is defined exclusively by the dimensions of one of the slits 9a .. . 9e and the slit 8. The apertures 13a . . . 13e merely serve to passthe radiation through one of the slits.

After the X-ray beam has also traversed the range defined by the slit9d, the slit 9e becomes operative for one half revolution, after theswitching off of the X-rays; subsequently the slit 9a becomes operativeetc. Thus, within five successive revolutions an X-ray beam emitted bythe radiation source traverses the axial length of the diaphragm body 3once. The apertures in the diaphragm body 12 may be distributed acrossits circumference in another manner, for as long as they are offsetthrough 180°/n on the circumference with respect to neighbouringapertures and are adapted to the dimensions of a slit in the axialdirection. However, spatially successive areas would not be successivelytraversed in time by the X-ray beam; instead the beam would jump, forexample from the strip 9c to the strip 9e or otherwise in a similarmanner.

The smaller the ratio of the diameter of the first diaphragm body 3 tothe distance between the radiation source 1 and the axis of rotation 5,the less noticeable the alternating widening and narrowing of the X-raybeam described above will be. In cases where such "breathing" of theX-ray beam can be tolerated, it is not necessary to deactivate theradiation source 1 every other half revolution of the diaphragm body 3.In this case the second diaphragm body shown in the FIGS. 6 and 8 couldalso be used, be it that the X-ray beam would then jump from the slit 9ato the slit 9d and further to the slit 9b etc. However, the seconddiaphragm body 12 may then have an essentially simpler construction. Inthat case it would suffice to provide only a single helical wide slitwhich extends from the top left to the bottom right in FIG. 8. The widthof the slit must be sufficient in order to allow for the X-ray beam tomove within one of the slits 9a . . . 9e, but it should not be so largethat two slits in the plane defined by focus 1 and axis can besimultaneously exposed to X-rays. In this case the first diaphragmaperture may also be provided with an even number of slits.

It is also possible to choose the diameter of the two diaphragm bodiesso that the first diaphragm body comprising the slits is situated withinthe second diaphragm body. The described "breathing" of the X-ray beamin the case of continuously applied X-rays is then less pronounced; whenthe X-rays, however, are switched off after every other half revolution,it is generally more effective to choose the reverse device (firstdiaphragm body on the outside) shown in FIG. 4, because the X-ray beamemerging from the slits is then better defined.

What is claimed is:
 1. A device for forming an X-ray beam or gamma raybeam of small cross-section and variable direction, comprising an X-raysource or gamma source which supplies a radiation beam and a diaphragmdevice which forms the X-ray beam from the radiation beam and whichcomprises a stationary diaphragm section provided with a rectilinearslit and a cylindrical first diaphragm body which rotates about an axisof rotation and which is provided with a helical slit on its outersurface, characterized in that the diaphragm body has an approximatelysemi-circular cross-section over at least a part of its length.
 2. Adevice as claimed in claim 1, characterized in that the axis of rotationof the diaphragm body is situated in the plane defined by the radiationsource and the rectilinear slit.
 3. A device as claimed in claim 1,characterized in that the helical slit has a pitch which differs overthe length of the diaphragm body.
 4. A device as claimed in claim 1,characterized in that the slit is stepped.
 5. A device as claimed inclaim 1, characterized in that a plurality of helical slits succeed oneanother in the axial direction in the first diaphragm body.
 6. A deviceas claimed in claim 5, characterized in that there is provided a seconddiaphragm body which comprises at least one aperture and which has asemi-circular section over at least a part of its length, the twodiaphragm bodies being coaxially arranged so that one body encloses theother, the first diaphragm body rotating at a speed which is higher thanthat of the second diaphragm body as a function of the number of slitsprovided therein, the arrangement and the shape of the aperture on thecircumference of the second diaphragm body being such that a usable beamcan emerge through each time only one of the slits.
 7. A device asclaimed in claim 6, characterized in that the first diaphragm bodyencloses the second diaphragm body.
 8. A device as claimed in claim 1,characterized in that the first diaphragm body is arranged between theradiation source and the diaphragm section (7).
 9. A device as claimedin claim 6, characterized in that in the second diaphragm body there areprovided n apertures n being the number of the slits in the firstdiaphragm body, the apertures being offset through an angle of 180°/nwith respect to one another on the circumference, their axial positioncorresponding to the axial position of a respective a slit so that theradiation each time passes through one of the slits and through theassociated aperture.
 10. A device as claimed in claim 6, characterizedin that in the second diaphragm body there is provided a sole aperturein the form of a helical slit which is substantially wider than theslits in the first diaphragm body and which extends through acircumferential angle of at least approximately 180°.
 11. A device asclaimed in claim 7, characterized in that in the second diaphragm bodythere are provided n apertures, n being the number of the slits in thefirst diaphragm body, the apertures being offset through an angle of180°/n with respect to one another on the circumference, their axialposition corresponding to the axial position of a respective slit sothat the radiation each time passes through one of the slits (forexample, 8b) and through the associated aperture.
 12. A device asclaimed in claim 2, characterized in that the slit is stepped.
 13. Adevice as claimed in claim 2, characterized in that a plurality ofhelical slits succeed one another in the axial direction in the firstdiaphragm body.
 14. A device as claimed in claim 2, characterized inthat the helical slit has a pitch which differs over the length of thediaphragm body.
 15. A device as claimed in claim 14, characterized inthat the slit is stepped.
 16. A device as claimed in claim 15,characterized in that a plurality of helical slits succeed one anotherin the axial direction in the first diaphragm body.
 17. A device asclaimed in claim 16, characterized in that there is provided a seconddiaphragm body which comprises at least one aperture and which has asemi-circular section over at least a part of its length, the twodiaphragm bodies being coaxially arranged so that one body encloses theother, the first diaphragm body rotating at a speed which is higher thanthat of the second diaphragm body as a function of the number of slitsprovided therein, the arrangement and the shape of the aperture on thecircumference of the second diaphragm body being such that a usable beamcan emerge through each time only one of the slits.
 18. A device asclaimed in claim 17, characterized in that the first diaphragm bodyencloses the second diaphragm body.
 19. A device as claimed in claim 18,characterized in that the first diaphragm body is arranged between theradiation source and the diaphragm section.
 20. A device as claimed inclaim 18, characterized in that in the second diaphragm body there areprovided n apertures n being the number of the slits in the firstdiaphragm body, the apertures being offset through an angle of 180°/nwith respect to one another on the circumference, their axial positioncorresponding to the axial position of a respective slit so that theradiation each time passes through one of the slits and through theassociated aperture.